Defines the log domain. See Log Domains.

Libraries should define this so that any messages which they log can be differentiated from messages from other libraries and application code. But be careful not to define it in any public header files.

Log domains must be unique, and it is recommended that they are the application or library name, optionally followed by a hyphen and a sub-domain name. For example, bloatpad or bloatpad-io.

If undefined, it defaults to the default NULL (or "") log domain; this is not advisable, as it cannot be filtered against using the G_MESSAGES_DEBUG environment variable.

For example, GTK+ uses this in its Makefile.am: |[ AM_CPPFLAGS = -DG_LOG_DOMAIN=\"Gtk\"

Applications can choose to leave it as the default `NULL` (or `""`)
domain. However, defining the domain offers the same advantages as
enum LOG_DOMAIN = 0;